Piezoelectricity has been used a number of useful applications. For example, crystal piezoelectric devices may be used for electronic frequency generation. With the increasing demand for miniaturization or weight savings in electronic devices, the tuning fork type crystal unit is becoming widely used. By forming the crystal unit to a tuning fork shape, the unit may produce low Crystal Impedance (CI) values. The CI value is equivalent to the inline resistance value of a crystal oscillator.
The structure of a typical tuning-fork type crystal unit is illustrated in FIGS. 6 (a) and (b). FIG. 6 (a) is a perspective view of a tuning-fork type crystal piece 100 and FIG. 6(b) is a cross-sectional view taken from line A-A′ of FIG. 6(a). It should be noted that indications of excitations for electrodes on the surfaces of the tuning-fork type crystal unit are omitted for convenience.
As showed in FIG. 6(a), the tuning-fork type crystal piece 100 is comprised of a base unit 103 and arm units 101 and 102 extended from the base unit in a tuning-fork shape. On the surface and rear surface of the arm units 101 and 102, groove portions 101a and 102a are formed on the both surfaces in a longitudinal direction. Therefore, the cross-sectional view of the arm units 101 and 102 are substantially H-shaped as shown in FIG. 6(b). In contrast to flat-shaped crystal units, such a tuning-fork type crystal unit having two arm portions with a substantially H-shaped cross-section may raise the electromechanical conversion coefficient even when the crystal vibrating piece is downsized, thus allowing for the unit to maintain a low CI value.
In order to vibrate the tuning-fork type crystal piece 100 at a desired frequency, an excitation electric field may be applied to the arm units 101 and 102. For this purpose, grooved electrodes may be formed in a longitudinal direction in the grooved portions 101a and 102a, as well as on the side parts 101b and 102b having no grooved portion. Each grooved electrode in the side portions is also formed in the longitudinal direction.
In order to form an excitation electric field on the tuning-fork type crystal piece 100, a driving excitation current may be applied from the outside of the unit. For this purpose, a base electrode which connects an external current source may be formed on the surface of the base unit 103. A connection electrode which connects such a base electrode to the above-mentioned grooved electrodes or side electrodes may also be formed on the base unit 103. Furthermore, a connection electrode that connects a groove electrode to a side surface electrode may also formed on the base unit 103 (not shown).
As the excitation driving current is applied from the external current to the tuning-fork type crystal piece 100 through the above described electrodes, the driving current flows to the grooved electrodes and side electrodes of arm units 101 and 102, resulting in an electric field between the electrodes. As a result, both sides of the substantially H-shaped cross section of the arm unit expands and contracts relative to one another due to piezoelectric effects, and the arm units 101 and 102 vibrate in the reverse flexural directions. Due to the flexural vibrations, the tuning-fork type crystal piece 100 may be a high Q value resonant element, wherein the Q value is the frequency selectivity (not shown).
Referring next to FIG. 7, illustrated is an exemplary electrode layout of the tuning-fork type crystal piece 100. Note that the figure shows a surface of the tuning-fork type crystal piece 100 and a depiction of the flip side is omitted to avoid redundancy.
Referring to FIG. 7, 103e1 and 103e2 are base electrodes on the surface of the base unit 103. 101ae is a grooved electrode on the grooved portion 101a of the arm unit 101 and 102ae is a grooved electrode on the grooved portion 102a of the arm unit 102. 101be and 102be are side surface electrodes on side surfaces 101b and 102b of each arm. Furthermore, side surface electrodes 101be and 102be are formed near the edge of the forks along the sides. Side surface electrodes 101be and 102be have grooved electrodes along each arm unit such that each side surface electrode is connected on both sides of each arm.
The base electrode 103e1 is connected to the side surface electrode 101be through a connection electrode 103e3. The base electrode 103e1 is also connected to a grooved electrode 102ae through a connection electrode 103e5. Similarly, the base electrode 103e2 is connected to the side surface electrode 102be through a connection electrode 103e4, and the base electrode 103e2 is connected to a grooved electrode 101ae through the side surface 102be and a connection electrode 103e6.
The size of the tuning-fork type crystal unit having the substantially H-shaped cross section is typically very small. For example, when the resonance frequency of a unit is 32.768 kHz, the width of the arm units 101 and 102 may be miniaturized to about 0.1 mm and width of the groove electrodes 101a and 102a may be miniaturized to about 0.07 mm. In this example, when forming the electrodes on the surface of the arm units, the installation area of electrodes may be less than 0.015 mm wide.
In general, since a width of 0.01 mm is typically needed for an electrode to maintain a good conducting state, it is possible that the space between a grooved electrode and a side surface electrode may be allowed to be up to about 0.005 mm. Consequently, due to possible errors in the manufacturing process, there may be a high probability of contacting and short-circuiting the electrodes, thus resulting in a decrease of the product yield rate during the manufacturing process. Moreover, in order to avoid the contacting and short-circuiting of the electrodes, the accuracy of the etching during the forming process of the electrodes must be improved, resulting in a more complicated process due to the miniaturization of the mask patterns and raising the cost of production.
Japanese Unexamined Patent Application No. 2002-076827, No. 2005-229143, and No. 11-160074 address such problems. Japanese Unexamined Patent Application No. 2002-076828 discloses the placement of a space between the connection electrode and the grooved electrode for a side surface electrode and/or the connection electrode for a groove electrode. Japanese Unexamined Patent Application No. 2005-229143 discloses, for example, the preventing of contact of the electrodes by gradually expanding the space between a grooved electrode and a side surface electrode with tapering at the width of the grooved electrode. Japanese Unexamined Patent Application No 11-160074 discloses a convexity division wall at the bottom of the base part of the tuning-fork shaped arm unit of a tuning-fork type crystal unit (hereinafter “fork part”) to prevent contact or short circuiting of electrodes on the side surfaces on the inside of the arm unit.
In general, in order to form each electrode on the surface of the crystal unit, a metallic film may be formed on the surface of the crystal unit, for example, by vacuum based deposition or other means. Photo resist is coated on the metallic film and a photo etching process is performed to form the desired electrode pattern.
When forming an outline of a tuning-fork type crystal unit, etching is done to a space between both surfaces of the tuning-fork arms and surfaces. Because of the aeolotropy of etching, complicated formations are inverted on the fork part. When each electrode is formed on an element in such an outline formation, photo resist tends to accumulate near the bottom of the fork part due to its complicated formation, and solarization of ultraviolet during the exposure of the etching process may be insufficient due to the complicated formation. Therefore, unnecessary metallic film which should be removed during the etching process tends to remain at the fork part, and contact and short circuiting of electrode pattern can easily occur between electrodes near the fork part.
Because Japanese Unexamined Patent Applications No. 2002-076827 and No. 2005-229143 are directed to the prevention of short circuiting between electrodes by expanding the space of electrodes at a surface or a rear surface of the tuning-fork type crystal unit, the disclosures do not address an appropriate solution for the failure near the bottom of the fork part. Furthermore, Japanese Unexamined Patent Application No 11-160074 discloses a solutions to the failure near the bottom of the fork part, but special processing is needed during the outline etching process for the bottom of the fork part, resulting in a complicated manufacturing process and increased manufacturing cost.